Improve README with more precise comparisons with Promises.

[galileopy]

Changing the description of "equivalent with Promises" to "approximation with Promises",
Fixing an example in the bimap section to have the example that would actually work as bimap
Adding rejection branch in fold example with Promises, since then is almost like a foldChain overloaded monster.

[Avaq]

Besides the small nitpick below, all changes look good to me. If you'll make that change and squash your commits into one, we can merge it. After merging, I'll merge the 11.x branch back into master, updating the beta docs as well.

[Avaq]

Instead of using "the approximation", let's use "an approximation" across the docs, as there are most likely more than one approximation.

[Avaq]

Thanks! :)